A variety of routes for producing high purity isobutene can be employed industrially. The oldest is the sulphuric acid extraction process, but it is expensive and obsolete; it is known to be a contaminating process as waste acid is discharged. Further, the isobutene yield does not exceed 90%. The company ARCO uses tertio-butyl alcohol (TBA) dehydration, TBA being a by-product obtained from their propylene oxide production process. The isobutane dehydrogenation process was developed during the last few years as a result of the large and increasing demand for MTBE. However, that process can only be profitable with very large production capacities.
High purity isobutene production from cracking MTBE is as suitable for small capacities as for large capacities. Further, such a route benefits from the infrastructure generated by the increasing importance of ethers in reformulated gasoline. A number of refineries throughout the world have MTBE production installations, for example. Further, there is a global exchange market for MTBE. This means that the production of high purity isobutene from MTBE can readily be carried out anywhere in the world, in and remote from refineries.
The concept of producing isobutene by decomposing an ether, more particularly MTBE, has long been known, but prior art processes have suffered from certain disadvantages.
In the process developed by SUMITOMO described, for example, in European patent application EP-A-0 068 785, the MTBE decomposition reaction is carried out in the liquid phase, in the presence of a solid acid catalyst which is an ion exchange resin. Two product streams are obtained: isobutene and methanol. As described, isobutene is obtained directly overhead from a distillation column with no other purification step. The isobutene obtained contains a certain number of impurities, beginning with a small fraction of methanol which is azeotropically distilled from dimethylether (DME), which is a volatile compound formed by condensing methanol in the presence of an acid catalyst. It is probable that the purity of the isobutene is insufficient for use in, for example, the production of polyisobutene or other copolymers. Further, there is no apparent way of avoiding the accumulation of heavy impurities such as dimers of isobutene or methyl sec-butyl ether (MSBE), which in the long run results in a fatal reduction of product purity.
In the process developed by ERDOLCHEMIE, described in U.S. Pat. No. 4,409,421, for example, isobutene is purified by eliminating residual alcohol entrained with the tertiary olefin by adsorption. This method has the disadvantage of requiring regular regeneration of the adsorbent. Further, the problem of recovery of the major portion of the alcohol from the decomposition step is not solved.